Plain old telephone service (POTS), the standard telephone service that most homes that are connected to a public switched telephone network (PSTN) use, is not generally used for concurrent transmission of voice and data, as it is currently limited to transmission rates of up to 56.6 kbit/s.
Technologies such as digital subscriber line (xDSL) services are being developed to allow data, voice, and video to be transmitted simultaneously over standard twisted-pair copper wire telephone lines at multimegabit rates. xDSL is seen as an alternative to the more costly and time-consuming installation of high-speed fiber-based broadband communications networks.
Competing xDSL technologies include ADSL, RADSL, HDSL, SDSL, and VDSL. Each is best suited to a different application, with trade-offs made between signal distance and speed and with each having different transmission bandwidth configurations.
Asymmetric digital subscriber line (ADSL) is perhaps the best known xDSL variant, providing more transmission bandwidth "downstream" to a telephone subscriber than "upstream" from the subscriber. Downstream transmission rates range between 1.5 Mbit/s to a theoretical 8 Mbit/s, while upstream transmission rates range between 16 kbit/s to a theoretical 1 Mbit/s or more, each depending on line quality, distance, and wire gauge. Depending on environmental conditions, ADSL can transmit data at a rate of 1.544 Mbit/s over distances of up to 6,000 m using standard 24-gauge wire, and 8 Mbit/s over distances of 4,000 m or less. ADSL works by sending digital pulses in the high-frequency area of telephone wires. Since these high frequencies are not used by normal voice communications, ADSL can operate concurrently with voice communication over the same telephone wires.
Very-high-bit-rate digital subscriber line (VDSL) is the fastest xDSL technology. It can deliver downstream transmissions at a rate of 13 to 52 Mbit/s and upstream transmissions at a rate of 1.5 to 2.3 Mbit/s over a single wire pair. Unfortunately, VDSL has a maximum operating distance of only 300-1,500 m.
In digital subscriber line access multiplexing (DSLAM) telephone carriers join and split voice and data transmissions, employing digital subscriber line access multiplexers to send and receive these transmissions to/from subscribers over standard telephone lines using ADSL modems. Line or POTS splitters at the carrier alternately divide upstream ADSL transmissions, directing voice communications onto the public switched network and data streams to service providers such as Internet Service Providers (ISPs), and join voice and data streams for downstream transmission. A POTS splitter at the subscriber's location similarly joins and splits ADSL transmissions, with voice transmissions terminating at a telephone or other POTS apparatus and data transmissions terminating at an ADSL modem DSLAM is disadvantageous in that it is relatively expensive, generally requires professional installation of the POTS splitter at the subscriber's location and rewiring of the subscriber's telephone outlets to accommodate simultaneous data and voice transmissions, and is largely incompatible with future VDSL and "Fiber to the Curb" (FTTC) systems now being planned.